The present invention relates to the preparation and recovery of maleic anhydride. More particularly, the present invention is directed to improvements in processes and apparatus for the preparation of maleic anhydride in which the recovery of maleic anhydride from a gaseous reaction mixture includes absorbing the maleic anhydride in a solvent, and subsequently stripping maleic anhydride from the solvent to obtain crude maleic anhydride product.
Maleic anhydride is used alone or in combination with other acids in the manufacture of alkyd and polyester resins. It is also a versatile intermediate for chemical synthesis.
Maleic anhydride is prepared commercially by contacting a feed gas comprising molecular oxygen and a suitable hydrocarbon (e.g., n-butane or butene) with a vanadium-phosphorus-oxygen catalyst to partially oxidize the hydrocarbon and produce maleic anhydride. Hydrocarbons are converted to maleic anhydride by passing the feed gas through a tubular reactor containing a fixed bed of catalyst. The reaction product gas which is produced contains maleic anhydride together with oxidation by-products such as CO, CO.sub.2, water vapor, acrylic and acetic acids and other by-products, along with inert gases present in air when air is used as the source of molecular oxygen.
The prior art discloses a number of methods of isolating and recovering maleic anhydride from the reaction product gas. For example, the maleic anhydride can be recovered by direct condensation from the reaction product gas or by scrubbing the gas with water and dehydrating the resulting aqueous mixture by azeotropic distillation with xylene. However, due to increased product yields, the preferred method of recovery comprises selectively absorbing the maleic anhydride in a suitable solvent and subsequently stripping the maleic anhydride from the resulting absorption liquor to obtain crude product. U.S. Pat. No. 4,118,403 (White) discloses contacting the reaction product gas with an organic solvent in an absorber column so that the maleic anhydride, as well as some of the oxidation by-products, are absorbed in the solvent. The solvent comprises a dialkyl phthalate having 2 to 8 carbon atoms in each alkyl chain (e.g., dibutyl phthalate), and from about 0.5 to about 10 weight percent phthalic anhydride. Maleic anhydride is isolated from the rich absorbing liquor by stripping the liquor in a single step stripping operation. The solvent which has been stripped of maleic anhydride is then cooled and circulated back to the absorber.
Although processes like that disclosed by White have proven to be satisfactory in recovering maleic anhydride in high yields, prior art methods of this type have several disadvantages. By-products of the partial oxidation reaction such as acrylic acid tend to accumulate in the circulating absorbing solvent, along with other contaminants such as maleic, fumaric and phthalic acids, phthalic anhydride and polymeric tars formed in the absorption and stripping loop. The polymeric tars contain anhydride, acid and ester units and, during steady state operation, have an average molecular weight of about 30,000. The accumulated contaminants, especially the tars, cause plugging and fouling of the process equipment (e.g., heat transfer surfaces), and may have an adverse impact on maleic anhydride quality. Moreover, tar generation reactions tend to be autocatalytic, so that accumulation of the tars may also have an adverse effect on yields.
In order to prevent the build-up of contaminants, White discloses filtering the absorbing solvent after it has been stripped of maleic anhydride to remove fumaric acid. White also teaches distilling a slip stream of the stripped solvent to remove soluble tars. The filtered solvent and the distilled solvent are then recombined and recycled to the absorber. Although the techniques disclosed by White provide some control over contaminant concentration in the circulating solvent, the need to distill at least a portion of the solvent to prevent the build-up of high molecular weight tarry substances in the solvent increases energy requirements, adds to equipment costs, increases thermal degradation of the absorbing solvent and complicates the recovery of maleic anhydride.